Proximity fuze



June 20, 1967 BAKER 3,326,13

PROXIMITY FUZE Filed Nov. 22, 1949 5 Sheets-Sheet 1 r0 SIGNAL /r/ AMPLIFIER ELECTRONIC SWITCH CONTROL 6 AMPLIFIER 3-'- Load T0 0$c.#/ R

2 .OO2mfd z'do'on mfd FIG. 2

.004 mfd .0lmfd igher than I magohm INVENTOR. AMBROSE D. BAKER ATTORNEY I6 .002 i M r To Load June 20, 1967 A. D. BAKER PROXIMITY FUZE Filed Nov. 22, 1949 5 Sheets-Sheet 2 0.5 HIM-- 5 SOUIB WM J R 22 FIG. 7 INVENTOR.

AMBROSE D. BAKER ATTORNEY Osc. No.2

June 20, 1967 A. D. BAKER 3,326,130

Filed Nov. 22, 1949 PROXIMI TY FUZE 5 Sheets-Sheet 5 hi l ' 3 0.5 "HUT $6800. 1 S @SQUIB F I 8 INVENTOR.

AMBROSE D. BAKER BY /@@/Z4- ATTORNEY This invention relates to proximity fuse circuits, and more specifically to circuits that will safeguard the fuze from actuation by adventitious or intentional radio 1mpulses other than those to which the fuze is designed to respond.

The normal or intended operation of a proximity fuze consists in emitting a high-frequency radio signal from an oscillator that constitutes part of the fuze, receiving a reflection of said signal by the fuze when the latter is in the vicinity of a target, and heterodying the received s1gnal 1n the receiver circuit of the fuze to produce a low frequency resultant, which is then amplified to provide sufficient out put to cause the thyratron of the fuze to fire, and thus m1- .tiate detonation of the ammunition to which the fuze is attached, upon sufiiciently close approach of said fuze to the target.

In the operation of ammunition that depends on proximity fuzes for its ignition, premature detonation may occur 'upon the presence of electromagnetic radiation of a wave length to which the fuze would normally respond. This may occur purely accidentally; or, more probably, upon enemy attempts to actuate the fuze by intentional jamming, wherein radio-frequency radiation is swept through a wide frequency range in the hope that the response frequency would be included in said range and would thus detonate the ammunition before it approached within destructive range of its intended target. The present invention provides protection of the fuze against such premature operation, by requiring the simultaneous reception of several different frequencies in order to actuate the fuze.

An object of the invention is to provide an improved electronic means which is extremely difficult to actuate prematurely, as by adventitious radio impulses other than those to which it is designed to respond.

Another object is to provide two amplifiers in the circuits of a proximity fuze, so designed and connected that two impulses of different frequencies must act simultaneously to actuate the output tube of the fuze, to cause .the latter to fire.

thereof;

FIG. 3 is a circuit diagram of the control amplifier; FIG. 4 is a circuit diagram of the electronic switch, and

the thyratron circuit associated therewith;

FIGS. 5 and 6 are the equivalent circuits corresponding to the upper L and lower M circuits, respectively, of FIG. 4;

FIG. 7 is a composite circuit diagram, including the two amplifier circuits and the electronic switch, working in common into a single thyratron in the output; and

FIG. 8 is a slightly different composite circuit diagram, illustrating a modification that may be made in the connections shown in FIG. 7.

Referring first to FIG. 1, the circuits are seen to consist of a two-channel system, the first channel including a signal amplifier It and the second channel a control ampliraisin Patented June 20, 1967 tier 11, each of which in the embodiment disclosed herein comprises two stages, and will be described later. The two amplifiers feed their outputs to the thyratron 13 through a suitable electronic switch 12. Each channel also has its own corresponding oscillator, or transmitter, not shown here.

Each channel may have, for example, a sensitivity of approxmiately 5 millivolts at 400 cycles per second. That is, if the received signal after heterodyning with the corresponding oscillator, is below this value, the fuze will fail to operate, while any signal of 5 millivolts or more will sufice to cause the thyratron to fire and thus to detonate the ammunition to which the fuze is attached, this characteristic serving to prevent firing before the fuze is within effective range of the target.

However, the present circuit is so arranged that the thyratron will not fire unless both amplifiers are sufiiciently energized and moreover in such a way that the positive half-cycle of the input signal energy appears simultaneously at the input sides of both channels.

The signal amplifier, shown in detail in FIG. 2, may suitably be a two-stage, resistance-capacitance coupled amplifier. Increased sensitivity is attained by providing regenerative feedback from the anode circuit of the output triode V to the screen-grid circuit of the input pentode V This is an unusual feature, as it is conventional to feed the output energy back to the control grid. The reason for the new connection is that thereby one less resistor is required, which is an important matter, where the available space is extremely limited as it is in proximity fuze amplifiers. It should be remembered also that actually the space saving is doubled here, since each channel thus has one less resistor.

The input resistor R suitably 22000 ohms, in series with the shunt capacitor C of .004 mid, provides a lowpass filter. The anode by-pass capacitor C of .002 rnfd, working in conjunction with the anode-cathode resistance (R of triode V provides additional high frequency attenuation.

The capacitor C of .002 mfd. and the 0.5 megohm resistor R provide means for decreasing the low frequency sensitivity. A slight increase in the middle and upper range sensitivity, 350 to 600 cycles per second, was obtained by making C small, 250 mfd., but its capacitance was reduced only enough to provide the desired over-all response curve.

In addition to acting as an element of a filter circuit, the low resistance of R renders the circuit reasonably insensitive to such influences as residual gas-content and leakage of pentode V thus providing greater uniformity.

Grounding the filament or cathode of the output tube V instead of that of the input tube V in series therewith, provides a desirable change in the grid bias and results in a relatively great over-all increase in sensitivity. However, if for any reason it should become desirable to ground the filament of V instead of that of V the sensitivity could be restored partially by changing R from 0.5 megohm to 0.3 megohm, which would also necessitate reducing R to approximately 5 megohms.

Resistors R of 0.1 megohm and R of 0.5 megohm jointly act as a screen grid voltage divider. Resistors R of 1 megohm and R of 0.5 megohm provide anode loads for V and V respectively. Capacitor C of .002 mfd. acts merely as an output coupling capacitor.

The numerical values of the various circuit components mentioned in the present specification are merely illustrative, and are supplied solely in order to give an approxiiate idea of suitable magnitudes. For convenience, all .these values are found on FIGS. 2, 3 and 4 but are not repeated on FIGS. 7 and 8, each of which includes both the signal amplifier and the control amplifier in one composite circuit, to avoid crowding the drawing.

The control amplifier is illustrated in FIG. 3. A careful comparison of FIGS. 2 and 3 will show that apparently they are identical, with the exception of the reference numerals, which differ by 10 for corresponding resistors and capacitors in the two FIGS. This is in fact the case, but there is one very important exception: The series capacitor C in the output lead L of FIG. 2 has a capacitance of .002 mfd. whereas the corresponding capacitor C in lead M of FIG. 3 has a capacitance of .01 mfd. five times as great as that of C This difference is necessitated by the fact that the signal amplifier of FIG. 2 works into a load of 100,000 ohms while the control amplifier of FIG. 3 has a load impedance that exceeds one megohrn. Inasmuch as FIGS. 2 and 3 are identical in other respects, no additional description of FIG. 3 would seem to be necessary.

FIG. 4 shows the electronic switch and the thyratron circuit associated therewith. When there is no voltage between either output lead L or M and the ground, a normal grid bias, say negative 6.5 volts, will exist at the grid of the thyratron V If now a signal be applied to M alone, the circuit will act as a half-wave rectifier, causing an average increase in the grid bias, with its peak minimum only a little less than said minus 6.5 volts.

A similar result will be obtained if a signal be applied to L and none to M. In the last-named case, however, the average increase will not be as great, due to the countervoltage developed across resistor R This is relatively unimportant, inasmuch as the presence of resistor R renders the circuit insensitive to this counter-voltage.

As a numerical example, let it be assumed that the rectifier circuit is even 100% efilcient and that R is ten times R Under these conditions, the minimum grid swing will reach a peak of negative 6.5 volts plus 0.1 times the peak value of the positive swing. If now the firing bias is negative 2.5 volts, the signal at L would have to reach 40 volts peak to permit ionization of the thyratron to occur. The design of the signal amplifier is such that this voltage can never be reached at L, due to the overloading which would occur long before then.

Thus a signal which appears at only L or M cannot initiate ionization of the thyratron, V On the other hand, should a voltage be applied simultaneously to both L and M, the voltage E (see FIGS. 5 and 6) will block rectifier action during its positive alternation, allowing voltage E if passing through its positive alternation, at that instant, to reduce the instantaneous bias voltage applied to the thyratron grid.

Provided the two voltages E and B are in phase, the following rule will apply:

(1) If E exceeds E the instantaneous voltage on the thyratron will be E E where E is the normal grid bias.

(2) If E exceeds E the instantaneous voltage on the thyratron will be E -E +0.1 E

The composite circuit shown in FIG. 7 comprises all the components shown in block diagram form in FIG. 1, illustrated in detail in FIGS. 2, 3 and 4, and already described individually. The upper half of FIG. 7 is the signal amplifier, the lower half the control amplifier, both of which jointly work into the thyratron circuit shown at the extreme right hand side of FIG. 7.

In this composite circuit, the various resistors and capacitors are designated by the same reference characters and are of the same magnitudes as in FIGS. 2, 3 and 4.

Inasmuch as the operation of the individual circuits has already been explained, this likewise is not repeated here. The only additional points to be noted as to FIG. 7 are that the resistors R and R are added, 22,000 ohms and 6,800 ohms respectively, and that a squib firing circuit is connected to the anode lead of thyratron V This circuit consists of a 0.5 mfd. capacitor C connected in series with a squib S, between the anode of V and the ground as shown. The high resistance of R namely, 2 megohms, prevents rapid charging of capacitor C and hence only a very small current will traverse squib S during 4 the charging period. However, when the thyratron V fires, the capacitor C which has been charged to substantially the full B voltage, suddenly discharges through the anodecathode circuit of the thyratron, whose resistance is relatively small when the thyratron is ionized, and the resulting current pulse is sufficient to ignite the squib.

In a modified composite circuit, illustrated separately as FIG. 8 but closely similar to that of FIG. 7, a few changes have been made. In FIG. 8 the circuits of FIG. 7 containing the capacitors C and C in series with the resistors R and R respectively, have been omitted, so that there is no longer any connection from the anode of V to the screen grid of V and from the anode of V to the screen grid of V and consequently no regeneration exists in these portions of the circuit. The resistor R has been tripled in value, to 300,000 ohms.

The other change also embodied in said modified composite circuit as shown in FIG. 8, is that the circuit portion P-Q replaces the corresponding portion PQ of FIG. 7. It will be seen that capacitor C and resistor R remain unchanged, but a new series resistor R has been added, and a selenium rectifier X acting as a limiter, has been bridged to ground from the connection between R and R Due to the omission of regeneration, this circuit does not have as sharp a frequency response as the FIG. 7 form, but in other respects the FIG. 8 operation is substantially the same as that of FIG. 7.

It will be clear from the above description of the connections and operation of the dual or composite fuze circuit that it would be extremely difficult to jam such circuit, because successful jamming would require the simultaneous presence of two radio-frequency signals of two definite pre-set frequencies, and moreover of a definite phase relationship to one another. Such signals are selfproduced in the normal operation of a fuze that incorporates the circuits of the present invention, but it can be seen readily that there would be a vanishingly small probability that they could be produced by jamming even if the enemy knew both frequencies of the fuze.

Obviously many modifications and variations of the present invention are possible in the light of the above teachings. It is therefore to be understood that within the scope of the appended claims the invention may be practiced otherwise than as specifically described.

What is claimed is:

1. In a proximity fuze, a first amplifier connected to a first signal source, a second amplifier connected to a second signal source, said first and second amplifiers having substantially the same electrical characteristics, a firing circuit including a thyratron and a squib connected to the plate of said thyratron, an impedance network interconnecting said amplifiers and said firing circuit, said impedance network including a resistor connected in series between the output of said first amplifier and the control grid of said thyratron, a rectifier having its anode connected to the control grid of said thyratron and its cathode connected to the output of said second amplifier, and a resistor network interconnecting the control grid of said thyratron and the output of said second amplifier, and a source of bias voltage having its negative terminal connected to an intermediate point in said resistor network and its positive terminal to the cathode of the thyratron.

2. In a proximiy fuze having a firing circuit including a thyratron and a squib connected in the plate circuit of said thyratron, means for preventing premature actuation of said firing circuit comprising, a first amplifier connected to a first signal source, a second amplifier connected to a second signal source, said amplifiers having substantially the same electrical characteristics, a resistor connected in series between the output of said first amplifier and the control grid of said thyratron, a rectifier connected between said grid and the output of said second amplifier, a resistor network connected between the output of said second amplifier and said grid, and means connected to an intermediate point in said resistor network for applying 5 a negative bias voltage to said grid relative to said cathode whereby said thyratron can be actuated only by the simultaneous positive alternation of the signal voltages from said amplifiers.

References Cited UNITED STATES PATENTS 1,504,303 8/1924 Aifel 250-10 1,635,959 7/1927 Round 179-171.3 2,230,483 2/1941 Cage 179-171.7 2,291,045 7 1942 Lancor 25 0-27 2,298,987 10/ 1942 Thomsen 179-171.3 2,312,139 2/1943 Weagant 179-171.7

6 Baldwin 179-171.7 Wales 102-702 X Thomas 250-10 X Frommer. Gaflney et a1. 250-20.52 X

BENJAMIN A. BORCHELT, Primary Examiner. JAMES L. BREWRINK, WILLIAM G. WILES,

Examiners.

10 A. GAUSS, F. M. STRADER, L. N. DAVIS,

W. C. ROCH, Assistant Examiners. 

2. IN A PROXIMITY FUZE HAVING A FIRING CIRCUIT INCLUDING A THYRATRON AND A SQUIB CONNECTED IN THE PLATE CIRCUIT OF SAID THYRATRON, MEANS FOR PREVENTING PREMATURE ACTUATION OF SAID FIRING CIRCUIT COMPRISING, A FIRST AMPLIFIER CONNECTED TO A FIRST SIGNAL SOURCE, A SECOND AMPLIFIER CONNECTED TO A SECOND SIGNAL SOURCE, SAID AMPLIFIERS HAVING SUBSTANTIALLY THE SAME ELECTRICAL CHARACTERISTICS, A RESISTOR CONNECTED IN SERIES BETWEEN THE OUTPUT OF SAID FIRST AMPLIFIER AND THE CONTROL GRID OF SAID THYRATRON, A RECTIFIER CONNECTED BETWEEN SAID GRID AND THE OUTPUT OF SAID SECOND AMPLIFIER, A RESISTOR NETWORK CONNECTED BETWEEN THE OUTPUT OF SAID SECOND AMPLIFIER AND SAID GRID, AND MEANS CONNECTED TO 